In order to reduce emissions, modern car engines carefully control the amount of fuel that is burned. The engines control the air-fuel ratio to achieve an optimum stoichiometric ratio. At the optimum stoichiometric ratio, all of the fuel is burned using all of the oxygen in the air. For gasoline, the stoichiometric ratio is about 14.7:1. In other words, for each pound of gasoline, 14.7 pounds of air is burned. The air-fuel ratio varies from the optimum stoichiometric ratio during driving. Sometimes the air-fuel ratio is lean (an air-to-fuel ratio higher than 14.7) and other times the air-fuel ratio is rich (an air-to-fuel ratio lower than 14.7).
The primary emissions of a car engine are nitrogen, carbon dioxide and water vapor. Air is approximately 78 percent nitrogen (N2) gas. Most of the nitrogen passes through the car engine. Carbon dioxide (CO2) is produced when carbon in the fuel bonds with the oxygen in the air. Water vapor (H2O) is produced when hydrogen in the fuel bonds with the oxygen in the air.
Because the combustion process is never perfect, some additional harmful emissions are also produced by car engines. Carbon monoxide (CO), a poisonous gas that is colorless and odorless, is produced. Hydrocarbons or volatile organic compounds (VOCs), resulting from unburned fuel that evaporates, are produced. Sunlight breaks these emission down to form oxidants that react with oxides of nitrogen to cause ground level ozone (O3), a major component of smog. Oxides of nitrogen (NO and NO2, together called NOx) contribute to smog and acid rain and cause irritation to human mucus membranes. Catalytic converters are designed to reduce these three harmful emissions.
Most modern cars are equipped with three-way catalytic converters. “Three-way” refers to the three harmful emissions that catalytic converters help to reduce—carbon monoxide, VOCs and NOx. The catalytic converter uses two different types of catalysts, a reduction catalyst and an oxidization catalyst. Both types include a ceramic structure that is coated with a metal catalyst, usually platinum, rhodium and/or palladium. The catalytic converter exposes the catalyst to the exhaust stream while minimizing the amount of catalyst that is required due to the high cost of the catalyst materials.
There are two main types of structures that are used in catalytic converters—honeycomb and ceramic beads. Most cars today use a honeycomb structure. The reduction catalyst is the first stage of the catalytic converter that typically uses platinum and rhodium to help reduce the NOx emissions. When the NOx molecules contact the catalyst, the catalyst separates the nitrogen from the molecule, holds on to the nitrogen and frees the oxygen in the form of O2. The nitrogen bond with other nitrogen that are also held by the catalyst, forming N2. For example:2NO=>N2+O2 or 2NO2=>N2+2O2
The oxidation catalyst is the second stage of the catalytic converter that reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. The oxidation catalyst reacts the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example:
 2CO+O2=>2CO2
The third stage is a control system that monitors the exhaust stream and uses the information to control the fuel injection system. Typically an oxygen sensor is mounted between the engine and the catalytic converter. The oxygen sensor senses oxygen in the exhaust. An engine control system increases or decreases the amount of oxygen in the exhaust by adjusting the air-fuel ratio. The engine control system makes sure that the engine is running at close to the optimum stoichiometric ratio and that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and CO.
While the catalytic converter reduces pollution, the catalytic converter can still be improved substantially. The catalytic converter only works at a fairly high temperature. When a car is started, the catalytic converter does not reduce the pollution in the exhaust until the catalytic converter reaches a predetermined temperature that is also called the light off temperature.
One conventional solution to the problem is to move the catalytic converter closer to the engine. The hot exhaust gases reach the catalytic converter more quickly and heats it up faster. This approach tends to reduce the life of the catalytic converter by exposing it to extremely high temperatures. Most carmakers position the catalytic converter under the front passenger seat, far enough from the engine to keep the temperature down to levels that will not harm it.
Preheating the catalytic converter is another conventional way to reduce emissions. The easiest way to preheat the converter is to use electric resistance heaters. Unfortunately, the 12-volt electrical systems on most cars do not provide enough energy to heat the catalytic converter fast enough. Most drivers will not wait several minutes for the catalytic converter to heat up before starting their car.